Recent progress in laser additive manufacturing of aluminum matrix composites

doi:10.1016/j.coche.2020.01.005

Current Opinion in Chemical Engineering

Volume 28, June 2020, Pages 28-35

https://doi.org/10.1016/j.coche.2020.01.005 Get rights and content

Highlights

•
Powder preparation methods of aluminum matrix composites are discussed.
•
Nucleation and growth mechanism of composite grains are analyzed.
•
Strengthening mechanisms of composites performances are explained.
•
Existing problems and promising directions for future research are considered.

Laser additive manufacturing (LAM) can produce high-performance and near net shape parts of aluminum matrix composite (AMC) with higher specific strength, better wear resistance and more outstanding physical properties than aluminum alloys, which are widely used in automotive and aerospace fields. This article covers emerging researches on particle-reinforced AMCs fabricated by LAM techniques. The current research status is reviewed from the perspectives of powder preparation, microstructure characterization and mechanical properties. The microstructure evolution of AMCs is discussed in depth and the formation mechanism of in situ phase is analyzed. In addition, different strengthening mechanisms of AMCs are studied in detail. The last part summarizes the merits and demerits of AMCs and proposes the existing problems and future research directions.

Graphical abstract

Introduction

Because of low density, high specific strength and good corrosion resistance, aluminum alloys are favored by aerospace, automotive and military fields [1, 2, 3, 4]. However, aluminum alloys also exhibit disadvantages such as poor electrical conductivity, high thermal expansion coefficient, and poor wear resistance [5, 6, 7]. Aluminum matrix composites (AMCs) are a new substitute for aluminum alloys, which display excellent specific stiffness, extraordinary wear resistance, outstanding structural integrity and physical properties, attracting the attention of researchers in various industries [8,9]. Combining high strength, thermal stability, ductility and isotropy, particle-reinforced AMCs are manufactured in a variety of ways and increasingly used as structural materials [10]. In the current study, the reinforced particles include ceramic particles (such as SiC, TiC and TiB₂), carbon nanotubes (CNTs) and graphene, and so on.

With high energy density, high processing precision, short processing cycle and personalized production, laser additive manufacturing (LAM) could realize fast near-net shape and full densification of high-performance metal parts [11]. Combining the advantages of aluminum alloy, metal-based composite materials, laser and additive manufacturing, AMC specimens prepared by LAM can be obtained in a short period with high forming accuracy and controllable mechanical properties. Furthermore, new functional materials for different industries can be prepared by changing the type of aluminum alloy, the reinforced particle type and laser energy density. On the basis of rapid fusing of materials, the microstructures of AMCs are fully refined, and the obtained components reveal ultra-fine grain and mechanical properties significantly higher than those of LAMed aluminum alloy and AMCs prepared by traditional methods.

This review focuses on the current research status and development of LAMed AMCs. The powder preparation method, microstructure characterization, mechanical properties and strengthening mechanism of AMCs are reviewed to provide a deeper understanding for LAM technology of AMCs. Finally, some problems and the development trends of AMCs specimens prepared by LAM are proposed.

Section snippets

Preparation methods of AMCs powders

The ideal composite powders present uniform element distribution and good fluidity. So far, common preparation methods for composite powders include ball-milling, powder electrostatic self-assembly and in situ reaction preparation.

Ball-milling mixing is the most common method [12,13]. Mixed powders and hard grinding balls are placed in a ball-milling jar for mixing. During the milling process, the balls collide with each other and the powders between the balls are bonded. This method can

Microstructure of aluminum alloys and AMCs

In the LAM processing, the aluminum powder in the laser irradiation region is melted in a very short time accompanied by nucleation and directional growth of the grains. The microstructure of the LAMed aluminum alloy is typical columnar crystal (Figure 1b) because of a lack of heterogeneous nucleation sites and the maximum temperature gradient along the building direction [16, 17, 18]. Under the action of laser, Marangoni convection occurs in the molten pool which promotes the uniform

Mechanical properties of AMCs

Because of grain refinement and the action of second phase particles, AMCs exhibit superior mechanical and physical properties over aluminum alloys, including hardness, wear resistance, tensile properties and electrical conductivity. Nano-AMCs present super fine grains and diffused second nano-phases, effectively increasing the length of grain boundaries and hindering the movement of dislocations. The unreacted particles are uniformly distributed in the AMC, which play a role of dispersion

Strengthening mechanism of AMCs

The strengthening mechanism of LAM specimens of AMCs can be mainly divided into the following:

(1)
Dislocation strengthening mechanism. Since the thermal expansion coefficient (TEC) of the aluminum is higher than that of the ceramic particles, high-density dislocations due to plastic deformation caused by thermal expansion accumulate in the grains and cross-tangle, resulting in increased deformation resistance of grains and excellent strength property. Therefore, the deformation resistance

Summary and outlook

LAM breaks though the traditional manufacturing methods, effectively prepares high-performance AMC parts with high density, high strength (equal to low carbon steel) and wear resistance, and significantly improves manufacturing efficiency. The AMCs prepared by LAM are good candidate methods for automotive and aerospace.

Micro-particle reinforced AMCs have higher hardness and wear resistance than aluminum alloys, but there is stress concentration at the reinforced particles and the influence on

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Acknowledgements

This work was supported by Shandong Provincial Key Research and Development Programunder the Grant (No. 2018GGX103026), Shandong Provincial Natural Science Foundationunder the Grant (No. ZR2017MEE042), National Key Research and Development Program of Chinaunder the Grant (No. 2018YFB1107900).

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Laser additive manufacturing (AM) of reaction-based metal matrix composites (MMCs) involves highly complex and non-equilibrium material transformation behavior, including melting, dissolution, precipitation, and solidification. Yet, the dynamics and interplay of these phase transformation processes remain poorly understood, posing substantial challenges in identifying the microstructure formation mechanism, and predicting and controlling the microstructure in the printed parts. Here we performed the in-situ X-ray diffraction experiment to characterize the phase evolution dynamics of the 316L + 10 vol.%TiC system during laser melting, which provides direct and quantitative insights of the complex phase reaction and evolution dynamics under rapid heating and cooling conditions relevant to additive manufacturing of reaction-based MMCs. Further in-depth thermodynamic and kinetic calculations revealed that most of the phase evolution behavior observed in the in-situ X-ray diffraction experiment cannot be solely explained by widely used equilibrium thermodynamic models, and diffusion-controlled nonequilibrium dissolution and precipitation kinetics must be considered to elucidate the complex phase evolution behavior, including incomplete TiC dissolution, and three-step TiC precipitation. The three distinct types of precipitates generate unique hierarchical TiC micro- and nanostructures, which enhances the yield strength from 513 MPa to 877 MPa by 71 %, tensile strength from 628 MPa to 1054 MPa by 68 %, and Young's modulus from 193 GPa to 221 GPa by 14 %. The findings of our research provide the knowledge foundation for the design of unique microstructures and advanced MMC materials through laser AM.
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Laser additive manufacturing (LAM) has attracted significant interest in developing Al alloys due to its potential to fabricate structural components at low cost and high efficiency. Nevertheless, LAM Al alloys still suffer many issues, including heterogeneous microstructure, various defects, and poor mechanical properties, significantly hindering the advancement of LAM Al alloys. This article seeks to provide a comprehensive overview of the current advancements made in the field of LAM Al alloys and attempts to establish an in-depth relationship between microstructural characteristics, defects, and performances. Development status and limitations of LAM technologies are described, microstructural characteristics of Al alloys in LAM are introduced, mechanisms of formed defects and corresponding mitigation measures are discussed, and current quality improvement strategies are summarized. Following the research of previous work, this review will provide an outlook on overcoming the existing challenges and provide a helpful resource and reference for the development of LAM Al alloys.
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Powder-based additive manufacturing (AM) is revolutionizing the fabrication of advanced engineering metallic materials, including aluminium (Al) alloys, which are the workhorse materials in automobile and aerospace industries. However, challenges remain in the wider applications of AM to produce Al components due to the high tendency to form coarse, textured columnar grains, which causes hot-cracking and severe property anisotropy. The recent adoption of inoculation treatment in AM of Al alloys has been successful in achieving grain refinement, cracking elimination and property improvement, which is a step forward in this field. This paper surveys the emerging researches on inoculation treatment of AM-fabricated Al alloys and provides a comprehensive overview of different inoculation techniques for AM, the refining efficiencies of various inoculants and their underlying mechanisms. The uniqueness of this review includes substantive discussions on the mechanism of epitaxial grain growth during AM and a succinct comparison of the refining efficiency based on both experiment and crystallographic modelling. Critical challenges in the most recent alloy design strategy embedded with inoculation treatment are also discussed. Accordingly, outlooks for the immediate future in this area, gaps in the scientific understanding, and research needs for the expansion of AM in fabrication high-performance Al alloys are provided.
Effect of internal defects on tensile strength in SLM additively-manufactured aluminum alloys by simulation
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This research investigates the effect of internal defects on the tensile strength of Selective Laser Melting (SLM) additively-manufactured aluminum alloy (AlSi10Mg) test parts used for civil aircraft light weight design. A Finite Element Analysis (FEA) model containing internal defects was established by combining test data and the stress concentration factor comparison method. The effect of variation in the number, location and shape of defects on the finite element results was analyzed. Its results show that it is reasonable to use spherical defect modeling. The finite element modeling and analysis methods are also applied to the study of the effect of internal defects on tensile strength in additive manufacturing of other metallic materials. According to the FEA results of single defects at different scales, the formula for calculating the weakening degree of tensile strength applicable to the defective area of less than 15% was established. The result of the procedure is reliable and conservative. This research results can guide the selection of process parameters for the additive manufacturing of aluminum alloys. Further, the research results can promote the application of metal additive manufacturing in designing light-weight civil aircraft structures.
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This study investigated the effects of solution treatment (ST) on the microstructure and mechanical properties of (SiC+TiB₂)/Al–Zn–Mg–Cu composite produced using laser powder bed fusion (LPBF). Microstructural analysis reveals that the grid-like distribution of eutectic precipitates within the as-printed composite disappeared, giving way to the formation of uniformly distributed precipitates. As the ST temperature increased, the content of spherical Si particles decreased, while the content of elongated Al₄SiC₄ precipitates increased. Matrix grains recrystallization occurred, resulting in a gradual increase in the average grain size from 3.79 μm to 5.23 μm. The preferred crystallographic orientation features along the printing direction, <001> {001} plate texture, were weakened, gradually exhibiting a more disordered growth. The microhardness and tensile strength of the composites exhibited a slight decrease and exhibited a trend of initially increasing and then decreasing with increasing ST temperature. Additionally, the heterogeneity in microhardness along different directions has been reduced. When the ST temperature was 460 °C, the elongation of the composite experienced a significant increase, escalating from 6.12 ± 0.36% to 10.3 ± 0.25%. However, with the increase in ST temperature, it displayed an obvious downward trend. The maximum tensile strength was 449 ± 8 MPa, and the elongation was 9.6 ± 0.4% with the ST temperature of 500 °C. The variations in mechanical properties were primarily attributed to the elimination of grid-like substructure, the uniform distribution of precipitates, and the gradual coarsening of grains. This research was expected to contribute to developing advanced lightweight materials with improved performance, opening up new opportunities for their application in various industries.
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Citation Excerpt :
On the other hand, aluminum alloys with a good L-PBF processability, i.e. the Al-Si and Al-Si-Mg based alloys [16,29], exhibit a yield strength (YS) of 225–310 MPa, an ultimate tensile strength (UTS) of 270–440 MPa, and an elongation at fracture of 4–10 % in as-built condition and after ageing heat treatments. However, these mechanical properties are not satisfactory for structural applications in automotive and aerospace applications [30,31]. There is therefore a need to develop high strength aluminum alloys with modified compositions, tailored to the L-PBF process conditions, to achieve advanced performance and high structural integrity.
Laser Powder Bed Fusion (L-PBF) provides great advantages in creating supersaturated solid solutions due to its intrinsic ultrafast cooling and high solidification rate, which is particularly desired for enhanced solid solution strengthening and precipitation strengthening. In the present work, a novel high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy with a good L-PBF processability is investigated. The as-built alloy exhibits a heterogeneous microstructure featuring a bi-modal grain structure with coarse columnar and fine equiaxed grains and heterogeneously distributed nanometer and submicrometer-sized cuboid-shaped primary Al₃(Sc,Zr,Hf) particles. The Al₃(Sc,Zr,Hf) phase, exhibiting a homogeneous elemental distribution of Zr, Sc, and Hf, adopts a cubic L1₂ structure with a lattice parameter of 0.4044 ± 0.009 nm, showing a good coherency with α-Al. Additionally, the rapid L-PBF solidification enables the formation of submicrometer-sized elongated and globular primary Al₆Mn and a supersaturated Al matrix with a high dislocation density. This results in a good combination of strength (yield strength of 438 ± 3 MPa and ultimate tensile strength of 504 ± 2 MPa) and ductility (elongation at fracture of 10.9 ± 1.4 %), with a moderate work hardening strength of 113 ± 12 MPa. Direct-ageing at 325 °C for 10 h promotes the formation of a large amount of rod-shaped Al₆Mn precipitates and a few spherical Al₃(Sc,Zr,Hf) nanoprecipitates. The heat treatment increases the hardness from 166 ± 2 HV in as-built condition to 173 ± 4 HV and enhances the tensile strength (yield strength of 487 ± 2 MPa and ultimate tensile strength of 542 ± 3 MPa) but slightly reduces the ductility to 7.4 ± 0.7 %. The high strength was achieved by the synergistic effect of grain boundary strengthening, solid solution strengthening, and Orowan strengthening mechanisms.

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Recent progress in laser additive manufacturing of aluminum matrix composites

Highlights

Graphical abstract

Introduction

Section snippets

Preparation methods of AMCs powders

Microstructure of aluminum alloys and AMCs

Mechanical properties of AMCs

Strengthening mechanism of AMCs

Summary and outlook

Conflict of interest statement

References and recommended reading

Acknowledgements

J Mater Sci Technol

J Vac

Procedia Struct Integr

J Mater Today Proc

J Procedia Manuf

J Opt Lasers Eng

J Mater Res Technol

J Mater Today Proc

J Mater Res Technol

J Opt Laser Technol

J Mater Sci Eng A

J Ceram Int

J Alloy Compd

J Mater Process Rep

J Powder Technol

J Compos Mater

J. Materials Science and Engineering: A

J. Carbon

J Mater Sci Eng A

J Alloys Compd

J Mater Charact

J Compos Part A Appl Sci Manuf

J Nano Lett